The present invention relates to a ceramic heater used mainly in the semiconductor industry.
Semiconductors are very important goods necessary in various industries. A semiconductor chip is produced, for example, by slicing a silicon monocrystal into a given thickness to produce a silicon wafer, and then forming plural integrated circuits and the like on this silicon wafer.
In the manufacturing process of this semiconductor chip, the silicon wafer put on an electrostatic chuck is subjected to various treatments such as etching and CVD, so as to form conductor circuits, elements and so on. Also, a resin for a resist is applied thereto and then the resin is heated and dried.
For such heating, a ceramic heater is used. Heaters using a carbide or a nitride are disclosed in JP Kokai Hei 11-74064, JP Kokai Hei 11-40330 and so on.
However, through holes, into which pins or lifter pins for supporting a semiconductor wafer will be inserted, are usually formed in such a heater, and thus a problem that the temperature in the vicinity of the through holes drops is caused.
An object of the present invention is to provide a heater wherein its heating face has even temperature even if a through hole is made.
The present invention is a ceramic heater comprising: a ceramic substrate mainly in disc form; and a resistance heating element formed on the surface of the ceramic substrate or inside the ceramic substrate, the ceramic substrate being equipped with a through hole, wherein the wall face of the above-mentioned through hole has a surface roughness of Rmax=0.05 to 200 xcexcm based on JIS B 0601.
The inventors conducted eager researches. As a result, it has been found out that the reason why the temperature in the vicinity of a through hole in the prior art drops is that ambient gas rises through the through hole by being heated so that the temperature in the vicinity of the through hole drops. Thus, the inventors have come up with the idea that by making the inner wall of the through hole rough and generating turbulent flow in the ambient gas to cause the gas to remain and accumulate heat, a drop in the temperature in the vicinity of the through hole can be prevented.
The present invention is characterized in that the surface roughness of the wall face of a through hole is as follows: Rmax=0.05 to 200 xcexcm based on JIS B 0601.
Since the surface roughness of the wall face of the through hole is: Rmax=0.05 to 200 xcexcm, ambient gas tends to remain in the through hole so that heat is easily accumulated. Moreover, the temperature in the vicinity of the through hole does not drop since the contact area between the wall face and the ambient gas is small.
As this ambient gas, air, reactant gas (such as CF4), inert gas (such as nitrogen or argon and the like) and the like are desired.
Rmax is defined as follows according to JIS B 0601: when a portion of a sectional curve is extracted at a standard length and the extracted portion is sandwiched between 2 straight lines which are parallel to the average line of the extracted portion, an interval between the 2 straight lines is measured in the direction of longitudinal magnification of the sectional curve, and then the measured value is represented in micrometers. In other words, Rmax represents a difference in level between the highest mountain and the deepest valley in relation to the average line of the sectional curve.
Ra is obtained by integrating the absolute value of a curve of roughness by the measured length, dividing this by the measured length, and then representing the resultant value in micrometers. By integrating the absolute value of the curve of roughness by the measured length, the area of the portion surrounded by the average line and the roughness curve is calculated. Dividing this by the measured length means to obtain an average height when the mountains and the valleys of the roughness curve are made even.
In the present invention, Rmax is important. This is because the manner that air flows depends on a difference in level between the highest mountain and the deepest valley in relation to the average line of the sectional curve.
The surface roughness JIS B 0601 is more desired to be Ra=0.005 to 20 xcexcm. This is because, in the case in which unevenness is averagely made in the face of the through hole, turbulent flow tends to be caused in the ambient gas.
The ceramic substrate is desirably a carbide ceramic or a nitride ceramic. This is because these ceramics are superior in thermal conductivity.
The thickness of the ceramic substrate is desirably 50 mm or less. If the thickness is over 50 mm, the ambient gas can be caused to remain in spite of the surface roughness. In the present invention, the advantageous effect thereof is produced particularly in the ceramic substrate whose thickness is relatively thin, that is, 50 mm or less. The thickness is optimally 25 mm or less. This is because temperature controllability is superior since the heat capacity thereof is small.